It's around the $100 mark for the complete recoil spring assembly which includes the guide rod, buffer, spring and retainer. You don't need to change the whole assembly when changing springs unless the rod itself is damaged. All you need is the spring and maybe the buffer if it's in bad shape.

Fun things about buying a Glock that come to mind...
1. Having a spare RSA that only cost you 8-18 bucks in your bag.http://www.lonewolfdist.com/Products.aspx?CAT=157
2. Having a buddy that thinks something that costs more is better. Shooting better than your buddy at the range with your "cheap" Glock. :D

I have owned Sigs, HKs,Glocks,S&Ws, etc. The value of a product is up to the buyer. Regardless of what they pay if the buyer is happy does that make it overpriced. For you a Sig or Hk is. For me they are not. Neither opinion is a fact, but merely a decision we both made.